So, you've just gotten a brand new plastic model kit that has lots of clear parts and a cool interior, and you're thinking, "Wouldn't it be cool if I could light this up somehow, but I don't know how?" Is that what's troubling you, fella? Well, read on, and I'll try to help you out here. If you've never built a plastic model before, I recommend you build a few simple ones and get familiar with the basics before tackling a project like this.

First Things First:

Do not even think about lighting your model with tiny light bulbs. That's just SO 20th Century! Seriously, while LEDs are only a tiny bit more complex to use than light bulbs, the benefits are enormous:

LEDs use a fraction of the power that equivalent bulbs use.
Because of the above, they generate almost no heat (at least in the small sizes).
They are available in gorgeous pure colors as well as white, without having to paint or dye them.
They can be easily dimmed without giving yellowish light as dimmed bulbs do.
They come in many more shapes and sizes than bulbs do, including so tiny you wouldn't believe it!
They also come in flashing and flickering types, without using any external components.

Step 1: Plan Your Work, Work Your Plan

I'm going to be using several models I've built with lights to illustrate this article. Most of the photos are of Pegasus's incredibly good Nautilus kit. (Sadly, this is not the Nautilus from the 1954 movie. Jules Verne never drew a picture, so this is as good an interpretation as many...) You want this model! The Martian War Machine (From the 1953 movie War of the Worlds) is another good Pegasus kit that begs to be lit up.

Before you even pick up that Exacto Knife or cement bottle, you must decide a couple of things:

Decide on a power source. The voltage of your power source will dictate some of your LED wiring, specifically, what resistors to use.I usually use a six or nine volt supply, using either 4 or 6 AA batteries, respectively. The supplier on the next page sells LEDs that already have the proper resistor for your power source.
Decide where you'll put the power source. I don't put the batteries inside the model, because that means you need a hatch in the model to get to the batteries, you'll also have to mount the switch on the model, and you'll have to handle the model every time you want to change batteries. I put the batteries and switch in a stand that the model is mounted to. For some models, such as cars, you might be able to put batteries and switch inside the model more easily.
Now that you've decided on a power source, shop for LEDs, especially if you're not familiar with their shapes and sizes. Decide where you're going to put them in your model, making sure there is enough interior room for wiring. (You may not be able to put wingtip lights on a plane, for instance, if the wings are too thin for the wires.) Decide whether the LED is going to be for area lighting, such as illuminating an interior (They'll need to be brighter for this), or spot lighting, meaning the LED is not intended to light something else. This may take some going back and forth, finding sizes and colors that will work for you, then figuring out if they'll fit.
If you're new to LEDs, read this Instructable for some tips.
Knowing how to solder is very helpful, but not absolutely necessary.

Step 2: Parts Is Parts...

What you'll need:

1. LEDs, of course! The number and size depends on your model, and your abilities.They come in several brightnesses. If you want them very bright, specify "Super Bright."
2. Resistors, unless you're a beginner and you buy the LEDs with the resistors pre-installed. See sources, below.
3. A battery holder or an ac "wall wart.". The minimum is 2 batteries (3 Volts), but you can use any reasonable voltage, depending on how long you want the batteries to last and how much room you have for them. Rule of thumb: Bigger batteries or more batteries= Longer battery life. If you use an AC adapter, all the same rules about voltages and resistors apply; you just have to find a place to put your incoming wire instead of batteries.
4. A switch, unless you feel like taking out batteries every time you want to turn it off. You probably want one that will stay on when you turn it on, such as a toggle, rocker, or slide switch. This will again depend on your space, taste, and budget.
5. Wire. Regular 22 gauge hookup wire from Radio Shack will do fine. Having 2 colors (Red and Black is good) helps since LEDs do not work when backwards.
6. A cool model kit you want to light up.
7. Assorted drill bits, knives, and the like to carve holes in your model for the LEDs.
8. Small pliers, cutters, and other modeling tools.
9. Hot glue gun to keep things neat.
10. (Optional, depending on skills and taste): Epoxy, a stand for the model, headphone jack and plug, soldering tools and solder.

If you need a primer on soldering, read the "soldering comic." It's the best simplest basic instruction I've found so far.

Parts Sources: Your parts sourcing will depend partly on how comfortable you are with electronics and LEDs. If you are inexperienced, you can get lots of prefabricated kits at http://www.modeltrainsoftware.com including microscopically tiny LEDs that will fit virtually anywhere. I like these guys the best for their helpfulness to beginners. If you use their LEDs, you can get by without soldering. Just twist all the wires together and tape or glue them to make them stay put. You don't have to figure out resistors, they do all that for you. They also sell the switches, battery holders, connectors, AC power supplies, and complete kits. They have some great tutorials on their site. You will pay more money, though.

You can get 10 or even 100 bare LEDs on eBay for what you'll pay for one LED here. And, of course, there's always Radio Shack. While they're not as good as they used to be, they still can be a handy (But expensive) source.

If you are comfortable working with LEDs and have, hopefully, a bit of soldering experience, buy your parts on eBay and save yourself lots of money.

Soldering Comic.pdf
Download

Step 3: Math? We Don't Need No Stinking Math!

This next step is for those who want to work with the bare parts. You can skim most of this if you've bought the prefab ones from the previous step. I am assuming here that you already know how to solder. You will first need to decide what resistor is needed for the LEDs. This will depend on your battery voltage, and to a lesser extent, the type of LED. A good rule of thumb, if you don't feel like doing math, is 100 Ohms (Brown-black-brown) for 3 volts, 470 Ohms (Yellow-violet-brown) for 6 volts, and 1,000 Ohms (Brown-black-red) for 9 or 12 volts. These are very conservative values. If you want more details, and you understand the units, use the LED calculator here.

LEDs are Polarized. This means that, unlike the switch and resistor, they care which way they are hooked up. The positive side of the LED has to be connected to the positive side of the battery, either through the resistor or the switch. If you wire one up backwards, it probably won't be damaged, but it also won't work. Study the first picture to see how to identify positive and negative. For other LEDs, the negative lead will always be the one marked in some fashion, either with a flat side, a notch, a dot, or something.

Solder the chosen resistor to one leg of the LED. It helps to make a small hook in the lead of each component so they'll stay together long enough to solder. It doesn't matter which leg, as long as you know which is which. Make a habit of always putting the resistor on the same leg (positive or negative), so it's easier. You will also probably need to add wires to the LEDs. Use 2 different colors (the standard is red for positive, black for negative) so you can tell positive from negative.

Step 4: Start Carving (The Model, Not the Turkey!)

Now that you have a bunch of LED sub-assemblies, whether you bought them or made them, they need to be hooked up in parallel. Refer to the first picture. This means you gather all the positive leads together and twist together, then all the negative leads likewise. These then get attached to the battery and switch circuit. Positive to positive, negative to negative.

I can't be too specific as far as mounting the LEDs goes; this will depend on your model and how elaborate you want to get. The first thing you want to do, though, is paint the entire interior of the model with a coat of black paint, preferably sprayed. Most of the plastic used for models passes light, and having light shine through the skin of the model will ruin the effect. Black paint is the most opaque color. If needed, once you have a good coat of black paint on, you can put silver paint or foil in there to reflect the light and make it more even.

You will need to make sure the wiring can be routed to the LEDs without being pinched, pulled, or otherwise molested. You may need to carve out channels for the wiring. A Dremel tool or sharp knife can help here. Depending on what you are lighting, you can either drill holes for the LEDs to mount from the backside, or just mount the LEDs inside with hot glue or epoxy. Test fit often to make sure you have clearance for the LEDs and wiring. On the models I've lighted, the LEDs are concealed behind clear parts, but if you're modeling something like, say, the red beacon on an old-style police car, sometimes you can just have the LED sticking out if it fits the scale of the model.

Prepare your chosen stand with the battery holder and switch, extending wires as necessary to get the power to your model. I've had very good success using a stand with a 1/4" or 1/8" headphone jack in the model, and the matching plug on the stand. This serves as stand and power connector in one; the bonus is, since the plug is round, the model will swivel on the stand! Install the connector in the model, if you're using one. Use lots of epoxy if you're installing a jack, but be careful not to get epoxy on the electrical parts.

If you're not going to use the plug-and-jack approach, determine where you're going to exit the wires from the model. Choose an inconspicuous spot and drill a hole just large enough for the wires to exit. They can be painted to match the model, or whatever color will make them blend in. Mask the wire ends, though, because electricity doesn't flow through paint, and you want to see your color code so you can hook it up correctly.

Step 5: This Is Only a Test...

I cannot stress enough the heartbreak you will experience if you do all this, glue your model together, and it doesn't work! So, test early and often.

You must test as you go, both for fit, function, and light leaks. Keep a lithium battery or your chosen battery pack handy to test function. Test also the fit, not only of the LEDs and wiring, but the model parts themselves. If the parts do not fit together snugly, you will have light leaks at the seams. If you can't make a part fit tight enough to block light, it can be puttied and sanded, or use one of my favorite tricks: Mix up a small batch of 5-minute epoxy, and then mix in some black acrylic paint to make the epoxy opaque. Work this into the seam, then wipe off the excess with a cloth dampened with rubbing alcohol. Neither the alcohol nor the epoxy will harm the plastic, as long as you get the excess off before it sets.

If you have any malfunctions, fix them now, before you go any further. This is the best reason to solder all connections... Soldered connections are much more reliable!

Bonus tip: When you are comfortable with LEDs, you can also adjust the brightness to suit your application. The resistor values I gave earlier are a good starting point, but if you need some LEDs brighter and some dimmer, you can increase the resistor value to dim them as much as you want. If, on the other hand, they're not bright enough, you'll have to be cautious. Too much current will blow the LED. For most small LEDs, 20 milliamps is about all you want to push through them. The only alternative is buy brighter LEDs. Search eBay for "Super Bright" LEDs, and you will find what you need.

Step 6: Take a Deep Breath...

The moment of truth!

Once you are absolutely sure (Test once more, to be on the safe side) everything works, finish joining all the model parts together.

Don't forget to mask off all your lights before painting!

Paint, detail, remove masking, and show it off!

Step 7: Go Win Some Contests!

I've won several prizes with these models. The only thing better than building them is seeing how they stack up in a contest. I encourage you to get involved with a local model club, learn from them, and share your experience!